Re: [PATCH] mm/hugetlb.c: fix unnecessary address expansion of pmd sharing

[Li Xinhai <lixinhai.lxh@gmail.com>]

On 1/1/21 1:56 AM, Mike Kravetz wrote:
> On 12/29/20 1:20 PM, Mike Kravetz wrote:
>> On 12/28/20 8:21 PM, Li Xinhai wrote:
>>> The current code would unnecessarily expand the address range. Consider
>>> one example, (start, end) = (1G-2M, 3G+2M), and  (vm_start, vm_end) =
>>> (1G-4M, 3G+4M), the expected adjustment should be keep (1G-2M, 3G+2M)
>>> without expand. But the current result will be (1G-4M, 3G+4M). Actually,
>>> the range (1G-4M, 1G) and (3G, 3G+4M) would never been involved in pmd
>>> sharing.
>>>
>>> After this patch, if pud aligned *start across vm_start, then we know the
>>> *start and vm_start are in same pud_index, and vm_start is not pud
>>> aligned, so don't adjust *start. Same logic applied to *end.
>>>
>>> Fixes: commit 75802ca66354 ("mm/hugetlb: fix calculation of adjust_range_if_pmd_sharing_possible")
>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>> Cc: Peter Xu <peterx@redhat.com>
>>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>>
>> Thank you.  That does indeed fix an issue in the current code.
>>
>> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> Upon further thought, this patch also expands the passed range when not
> necessary.  Consider the example (start, end) = (1G-6M, 1G-4M), and
> (vm_start, vm_end) = (1G, 1G-2M).  This patch would adjust the range to
> (1G, 1G-4M).  However, no adjustment should be performed as no sharing
> is possible.
>
correct, my previous patch did not fully fix the issue.

Above example maybe typo for vm_start, vm_end. The issue didn't fixed by 
my patch would be with another example,
(vm_start, vm_end) = (1G-8M, 1G+2M), (start, end) = (1G-6M, 1G-4M), end 
should not be adjusted to 1G, although after adjust it still below vm_end.

> Below is proposed code to address the issue.  I'm not sending a formal
> patch yet as I would like comments on the code first.  It is not a critical
> issue and any fix can wait a bit.
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 7e89f31d7ef8..41ccec617f74 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5264,16 +5264,19 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
>   	if (!(vma->vm_flags & VM_MAYSHARE))
>   		return;
>   
> -	/* Extend the range to be PUD aligned for a worst case scenario */
> -	a_start = ALIGN_DOWN(*start, PUD_SIZE);
> -	a_end = ALIGN(*end, PUD_SIZE);
> -
>   	/*
> -	 * Intersect the range with the vma range, since pmd sharing won't be
> -	 * across vma after all
> +	 * Check if start and end are within a PUD aligned range of the
> +	 * vma.  If they are, then adjust to PUD alignment.
>   	 */
> -	*start = max(vma->vm_start, a_start);
> -	*end = min(vma->vm_end, a_end);
> +	a_start = ALIGN_DOWN(*start, PUD_SIZE);
> +	a_end = ALIGN(*start, PUD_SIZE);
> +	if (range_in_vma(vma, a_start, a_end))
> +		*start = a_start;
> +
> +	a_start = ALIGN_DOWN(*end, PUD_SIZE);
> +	a_end = ALIGN(*end, PUD_SIZE);
> +	if (range_in_vma(vma, a_start, a_end))
> +		*end = a_end;
>   }
>   
>   /*
> 
Now, this fully fixed the issue.

One thing to be sure is that the (start, end) as input parameter must 
already within vma's range, although the range_in_vma test() can cover 
the out of range cases.

Reviewed-by: Li Xinhai <lixinhai.lxh@gmail.com>